Comando seems to be a hot topic so let's start with that first.
See, in my little numbskulled head, I picture Support Specialist, Commando and Firebug as combat classes. The Commando falls in the middle of the two, not as good as the firebug against crowds, not as good as the S.Spec against the big guys. He doesn't seem to excell against either, but does an average job for both, personally I find this a disadvantage, some may find it an advantage, but this is my pointless wall of text here.
Berserker has a good yet not great damage per second, not to mention he has to get up close and personal and go nuts, unless he has the common sense to bring a back up weapon. Despite this, he can hold his own pretty well seeing as he doesn't need ammo for melee weapons (Herpderp) and his great damage resistance, not to mention speed bonuses for getting out of sticky situations and kiting. Personally I find the class very fun.
Demolitions, suuuuure explosives cost alot, but come on, with this perk you're kill whoring like crazy! You probably have twice the income of your team mates by the end of a wave assuming you don't use the LAW. And really, a 1600 dps weapon with splash damage, extremely powerful proximity mines and to top it off you gain extra damage against fleshies? However as I stated in my overly edited comment before this, you can damage yourself, you have a small ammo capacity and a long reload time. It is nearly impossible to function in cqc. In my opinion, the downsides don't balance out the upsides.
And as for the Sharpshooter, I kinda summed it up earlier, good at taking down large specimens and good at taking down crowds when SPAMEBR happy. The only way I could ever see someone calling this class underpowered is if the game screws you over with the head hitboxes night and day, and that isn't even relative to the actual balacing of the game.
There, I've tried to explain things the best I can, now leave me alone and let me eat my mushrooms in peace.